


Knot One, Pearl Two

by TellTaleKael



Series: Knotty Boys [2]
Category: Undertale (Video Game)
Genre: Alpha Cross, Alpha/Beta/Omega Dynamics, Breeding, Enthusiastic Consent, Error Sans/Cross Sans, Error/Cross - Freeform, Knotting, M/M, Mating Cycles/In Heat, Multiple Orgasms, Omega Error, Restraints, Riding, Soulmates, they’re so in love, trying to get pregnant
Language: English
Status: Completed
Published: 2021-02-22
Updated: 2021-02-22
Packaged: 2021-03-12 12:28:04
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,073
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29634645
Author URL: https://archiveofourown.org/users/TellTaleKael/pseuds/TellTaleKael
Summary: Third time’s the charm, right?
Relationships: Sans/Sans (Undertale)
Series: Knotty Boys [2]
Series URL: https://archiveofourown.org/series/2177424
Comments: 15
Kudos: 138





	Knot One, Pearl Two

**Author's Note:**

> This is a sequel commissioned by [Paranormal Aries](https://twitter.com/peach_gamer?s=21) over on Twitter, who wanted to see these two boys again. :3

Error sighed, clutching his shirt in his hand, twisting it around. Two heats he had spent with Cross, knotting together five times overall. All that, and he still had yet to conceive. Sometimes, he wondered if Sci had been wrong; maybe it wasn’t even possible for him to carry. But no, the multitude of lingering tests had suggested no hint of infertility. By all accounts, it should be possible. 

Third times the charm, right?

Error had a good feeling about this time. Not because of some old idiom, no. If he was ever going to conceive, it would be this time. Error’s heats were unpredictable, irregular, and often sudden. But Cross? His ruts were always uniform. He knew exactly when he would fall into rut down to a span of a few hours. His rut was due...

...and Error’s body burned with preheat.

Opening up a portal, he stepped into the kitchen of Nightmare’s castle. Horror was there, preparing a cooler of drinks and easily-eaten finger foods. He turned his hood eyelight to Error, inhaling. As a beta, his senses weren’t as good as an Alpha’s, but there was no mistaking the scent of an omega going into heat. Especially Error’s, dark chocolate with the tang of old copper. Food and blood, two things that Horror was well versed in. 

“guess you’ll be spending th’ week with cross, then?” he rumbled. Error nodded. “good. y’ can help me carry th’ supplies up.” He wiped his hands and closed the lid of the cooler. He picked up a tray and passed it to Error. A peek showed that it was a pan of soup, something fresher for Cross to have before the worst of the rut hit, and all he could do was eat small bites. As their self-appointed chef, Horror always made sure everyone was well-fed, and none of them had the heart to turn away his offerings. It certainly helped during heats and ruts, having sustenance from a trusted source. 

Error followed Horror up to the residential wing, towards Cross’ little suite. He’d spent plenty of nights there with his mate, but he’d never been with Cross through a rut. As guilty as he felt, since Cross helped him through his heats, Error wasn’t always able to handle contact when Cross went into a rut. The most he could do was pass how worn clothes to his mate through a portal, for the alpha to press his face in to as he thrust into his own hand, trying to find relief. But not this time. This time, their cycles matched up. 

Horror stopped when they reached the door, looking at Error. He put the cooler down, and took the tray. “better let me ‘ave that,” he said, before knocking on the door. 

There was a muffled thump, and unsteady footsteps, and then the door swung open. “Thanks, Horror-“ Cross began, though his words trailed off as he looked up. Inhaling, his eyelights expanded.

In the blink of a socket, Cross had Error snatched up, put behind him so he was between the omega and Horror. A strained snarl left him, before he managed to force his shoulders to relax. “Sorry,” he told Horror, voice torn between guilt and anger. Horror waved his apology off.

“‘s fine. i expected it.” He nudged the cooler with his foot. “food fer ya and yer mate,” he said softly. “y’ need to make sure he has plenty to eat, yeah?” 

Appealing to Cross’ alpha side was enough to break him from his protective stupor, and he quickly grabbed the cooler up, bringing it inside. He eyed the tray in Horror’s hand, until it was passed over to him. He sniffed it over, apparently deeming it safe, holding it to his chest. 

“y’ both eat yer fill,” Horror continued calmly, not at all offended by Cross’ actions. “i can bring more up if needed.”

“Thank you,” Cross said, already nudging the door closed with his foot. “I owe you.” The door shut on Horror’s knowing chuckles, making Error flush.

Any embarrassment left when Cross put the tray aside and reached for Error. Even in a more instinctual mindset, he paused before touching his mate, waiting for Error’s permission. Error gave it with a nod, and across pulled him close. The alpha sniffed his neck, scenting him deeply. “You’re in heat,” he rumbled.

“Yes.” Error gripped Cross’ shoulders tightly. “And I’m here to spend it with you.”

Cross shuddered, backing Error against the door. He swallowed, pressing close, pinning Error with his body. “C-can I-?”

“Yesss,” Error hissed.

The bulge in Cross’ pants was hot, even through their clothes, as it pressed against Error’s thigh. Cross’ phalanges dug into the wood of the door, his breath coming out in grunts against Error’s neck as he humped the omega. Messy and desperate, he panted and licked Error’s bones. Error reached around, digging his phalanges into the lean muscles of Cross’ ass, pulling him in harder, rocking back against him in turn. 

They moved together until Cross stilled, gasping as he reached his peak. Error would have teased him for coming in his pants after a little grinding, if Cross hadn’t hit his knees. He tugged Error’s shorts down, not even bothering to let him step out of them completely before he had one leg thrown over his shoulders. Burying his face between soft, dark blue thighs, he didn’t come up for breath until Error had shuddered through his own orgasm on the alpha’s tongue. 

His legs threatened to give out, but Cross caught him on the way down, lifting him up and carrying him over to the bed. Error found himself spread out onto a rumpled, slightly off-center nest of blankets and pillows and clothes that all smelled strongly of Cross. He kicked his slippers off, sitting up and crossing his legs. Cross fluttered around the room, getting things set up and generally fussing. When he was finished, he joined Error along with the pan of soup. It seemed the quick orgasm had cleared his mind a little.

“We should eat this before it gets cold.” Horror had left them only one spoon, which Cross used to scoop some vegetables up and offer to Error. Error obediently accepted the bite. The carrots melted on his tongues, the potatoes fluffy and perfectly cooked. After three more bites, Error rolled his eyelights.

“You eat too,” he insisted, pushing the spoon back to Cross. When it looked like the alpha would protest, he continued, “You’re going to need the energy to satisfy my heat.”

Was it a little manipulative? Perhaps. But Cross began to eat as well, taking a bite for every few Error was given, which was better than nothing. Alpha ruts were definitely more instinctual and wild than omega heats, only happening once or twice a year. Cross was more level headed than was usual, but even he tended to get caught up in himself. 

“I didn’t know you would be in heat,” Cross mused around a bite of potatoes. Error shrugged.

“I didn’t either, until I woke up cramping this morning,” he said wryly. “Good timing, though.” 

Cross nodded vigorously, practically beaming. “You’re here,” he sighed. “I’m glad you’re here. I always think about you.” He reached over and took Error’s hand.

“Always?”

“Always.”

Error blushed and looked away. “Finish your soup,” he mumbled. Cross obeyed, lifting the pan and slurping up the last of the food. He put the pan aside without looking away from Error, so hopeful and happy that Error sighed and opened his arms. “Come here.”

Cross wrapped his arms around Error, kissing and cooing at him. His scent was thick with delight, and Error felt his own glans open more, allowing his own scent to join his mate’s, filling the air. He wasn’t surprised when Cross began to purr, the sound rumbling in his chest. The vibrations tickled Error until he couldn’t stand it anymore, laughing out loud. Cross looked at him with such a love struck expression that he couldn’t help but kiss the alpha again, trying to hide his blush against Cross’ face. 

“I think this will be the one,” Error said softly when the kiss ended. He cupped Cross’ face between his palms. “I have a good feeling about it this.”

“Yeah?” Cross’ eyelights shone.

“Yeah. So...” Error leaned back, pulling his shirt off, baring his body. Being in heat, around his mate’s rut, had him more developed than usual, breasts heavy on his chest. “I want you to breed me, alpha,” he breathed, sockets lidded and teeth curled up in a crooked smirk. 

For a split second, Cross was frozen. When his mind caught up, he pounced, pinning Error back down against the nest, teeth open in a snarling growl. Error tilted his skull back, baring his neck, and Cross’ eyelights burned hotter. He closed his teeth around the mating mark, worrying it with a gentleness that didn’t match the desperation in the rest of his movements. 

Reluctantly, he pulled back, sitting on his haunches to rid himself of his clothes. His shorts ripped in his fervor, but he didn’t seem to pay it any mind, throwing the ruined garment aside to join the rest in a pile on the floor. He was on Error in an instant, body pressed against him, hard and hot. His cock was painfully flushed, throbbing where it laid against Error’s thigh. He captured Error in a kiss, licking into his mouth and wrestling with his tongues for dominance. “Mine,” he growled between kisses. Error licked him back, humming.

“Yours,” he agreed, voice barely even glitching.

Cross nudged Error’s legs open with his knees, sparing a hand to lift one up to spread him even wider. He lined up with Error’s dripping entrance, rubbing the head between his folds teasingly. “Can I?”

“If you don’t, I’ll string you up and fuck myself on your cock,” Error warned.

Cross smirked. “Maybe next time.” He thrust in before Error could respond, hilting completely in one smooth motion. Error clenched down around him hard in retaliation, making him groan and drop his forehead down to press to Error’s. “Asshole,” he muttered, unbearably fond.

“No you,” Error gasped back, the cramps of heat demanding they get a move on. He twisted his hips, the movement making Cross’ give an involuntary jolt. “Alpha,” he practically whined. “Come on.”

“I’d rather come in, if it’s all the same to you.” Cross grinned at his stupid joke and Error just had to kiss him again. Thankfully, he finally decided to move, digging his phalanges into Error’s hips as leverage. Despite the earlier orgasm, the pace Cross set was fast and hard, the feedback loop of pheromones from alpha and omega in simultaneous cycles demanding he fuck and breed. Not that Error was complaining, clinging to Cross’ back so hard that he was sure he left scratches on the defined ecto. The sting didn’t even make the pace falter, Cross simply groaning and continuing on. 

Error reached his climax first, body tensing as hot sparks of pleasure raced up his spine. He cried out, sockets slamming shut as he arched up into Cross. The alpha fucked him through his orgasm, extending the pleasure as long as possible. When Error relaxed, slumping back against the nest, Cross took advantage of his relaxed state, chasing his own orgasm. His knot caught, locking them together and preventing any of the thick seed that flooded Error’s passage from spilling out. 

Error moaned at the heat that filled him, hand slipping between them to rest on his abdomen. One of Cross’ hands joined, and though they didn’t speak, it was clear that their thoughts were the same.

_ Come on. Take. Conceive. _

After a moment, Cross scooped Error up so he was sitting in the alpha’s lap. A soft kiss was pressed to his teeth as Cross’ arms held him tight. Little spurts of seed still burst from his cock, far more than he had ever produced before. It was enough to quell the cramping Error had been dealing with, at least for a little while, his body happy it was getting what it deemed necessary. 

“I love you,” Cross murmured, nuzzling against Error, unconsciously yet instinctively spreading his scent over his omega, chest rubbing against Error’s breasts and sending little sparks every time his nipples caught against corded muscles. 

“I love you too. Obviously,” Error said, blushing brightly. “Your mark wouldn’t have taken otherwise.”

Cross nuzzled his way over to said mark, lapping at the scar with his tongue. “I was so happy when it did,” he said softly. “I never thought you would accept me as a mate, but I’ll forever be grateful that you did. Nothing in my life has ever made me more proud than to be with you.”

Little glitches sparked over Error’s cheekbones, and he hid his face against Cross’ neck. “You’re the least terrible abomination to exist. I’m... excited to carry your children.” He couldn’t see it, but he could feel the dopey smile that spread on Cross’ face. 

With a huff, he pushed Cross back down on the nest. A flick of his phalanges sent strings to wrap around the alpha’s wrists and pull them out to the side. Cross didn’t look all that surprised, grinning up at Error. “Going to make good on your threat from earlier?”

“Damn right I am.” Error rocked his hips down, grinding the head of Cross’ cock against the back of his magic, making them both groan. Since it was the first knot of their cycles, Cross’ cock deflated not too long after, allowing Error to lift his hips up and slam back down, really riding Cross. Their thighs met with loud smacks and wet squelches, Error’s breasts bouncing and swaying in time with his movements, and Cross strained at the strings. Error tutted and tightened them down.

“Be a good boy,” he teased, sockets fluttering when he found the perfect angle to hit his sweet spot every time he dropped back down. “You’ll have your chance later... if I decide to let you go.” Cross groaned, flattening his feet on the bed so he could thrust up to meet Error. Error allowed it, chasing his pleasure on his alpha’s cock. He brought a hand down to pinch at his own clit, making his hips falter, before picking back up at a rapid pace. 

It felt like no time at all before Cross’ knot sank in and wouldn’t slide back out. Error came with more magic painting his insides, leaving his stomach stained a dark purple. His eyelights flickered, and he just managed to release Cross’ arms in time for the alpha to catch him as he slumped forward. 

“Mm,” he hummed tiredly. Cross kissed his temple, rolling them over to cuddle together on their sides.

“Rest,” Cross said, kissing him softly. “We can pick back up after a nap.”

“We better,” Error mumbled, closing his sockets. “Not stop-pping until I’m bred,” he managed to say around a yawn. There were still days left in their cycles, and plenty of chances for something to take. 

And it was going to be an enjoyable time, too.

***

Cross’ rut lasted a full seven days. It was seven days of messy fucking, rushed bites of food, loving words, and more fucking. Horror had to refill their supplies twice so they could keep their energy up. Error had never had a heat last so long, nor had he ever been able to handle touching so many consecutive days in a row. He was exhausted by the time their cycles cooled down, collapsing on the ruined bedding and falling asleep almost instantly. Cross curled around him, following him into unconsciousness. 

Error slept dreamlessly, waking after an untold amount of hours. He was sore from head to toe, ecto still summoned and sticky with sweat and fluids. Cross’ arm was thrown over his waist, the gentle snoring in Error’s acoustic meatus telling him that the alpha was still asleep. His bones rubbed against Error’s back with every breath; it seemed he had been able to banish his ecto already. 

Error was in no hurry to wake Cross up, as that would mean they would have to start cleanup, which would no doubt not be nearly as fun as making the mess had been. If he wasn’t so deeply, stupidly in love, he would have skipped out and left the mess for Cross, but he couldn’t bring himself to do it. 

Instead, he laid there in Cross’ arms, taking stock of his own body. His thighs and pussy were nearly numb, his ass, hips, and breasts covered in phalange-shaped bruises and bite marks. His mating mark had been opened again, Cross biting down on it several times. He was tired and worn out and oh so, so satisfied. He wasn’t even glitching much, even though his heat was over and Cross was touching him in many places. He decided to enjoy it while it lasted, instead of questioning it. 

Most of all, his soul was full of love for this alpha he had given his life over to.

Error curled his legs up towards his body, threading his phalanges through Cross’ and lightly playing with them. He never would have expected that he would ever be so... domestic. Or domestic-adjacent. He was still Error, the Destroyer. But waking up in his mate’s arms just hit a little differently.

Speaking of domestic, though...

Error lifted Cross’ arm, peering down. His breath caught in his throat.

Two little souls were visible in his abdomen.

“Twins?!” 

Error shot up, knocking Cross off of him. He ignored the alpha’s confused grumblings, clutching at his stomach. The souls were still there, and not a figment of his sleepy imagination. Cross sat up with a wide yawn, scratching his ribs.

“Whas’it?” he mumbled. Error turned to him, removing his hands. Cross’ arms dropped down to his sides and his sockets widened. “Error...”

“Twins,” Error whispered.

“We’re going to be parents?!”

Cross scooped Error up into a tight hug, kissing him all over his face. “We did it!” he cried. “We’re having babies! Plural!”

Error felt tears spilling out of his sockets, but he knew they were happy ones. He took Cross’ hand, placing it over his abdomen, where the little souls were pulsing slow but steady. Their little family was finally growing.

“We did it,” he repeated, smiling.


End file.
